Modularize the build.

1. Add -DLINE_SEARCH_MINIMIZER to CMake to make the line search
   minimizer optional.
2. Better handling of -DSUITESPARSE/-DCXSPARSE in top level cmake
   file.
3. Disable code which will never be used if SuiteSparse and/or
   CXSparse is not available.
4. Update build docs.
5. Update jni/Android.mk
6. Minor lint cleanup from William Rucklidge.

Change-Id: If60460a858000df82faed7a6bb056dd2bfdde562

internal/ceres/line_search_minimizer.cc

@@ -38,6 +38,8 @@
 // For details on the theory and implementation see "Numerical
 // Optimization" by Nocedal & Wright.
 
+#ifndef CERES_NO_LINE_SEARCH_MINIMIZER
+
 #include "ceres/line_search_minimizer.h"
 
 #include <algorithm>
@@ -281,3 +283,5 @@ void LineSearchMinimizer::Minimize(const Minimizer::Options& options,
 
 }  // namespace internal
 }  // namespace ceres
+
+#endif // CERES_NO_LINE_SEARCH_MINIMIZER